User interfaces are used to enter information into an electronic device. For example, pointing devices, such as a joystick, mouse, and trackball, are typically used to position a cursor on a screen. A mouse and a trackball typically use electro-mechanical or optical systems to convert a rotational motion of a ball to a linear motion of a cursor. Joysticks typically include an array of digital contact switches that detect when the joystick is moved in a particular direction.
More sophisticated analog pointing devices control the speed and direction of cursor movement by sensing the magnitude and direction of a force applied to the pointing device. For example, to use the Porta-Point.TM. and Dura-Point.TM. pointing devices sold by Interlink Electronics of Camarillo, Calif., a computer operator presses an elastomeric pad that covers an array of four force-sensing resistors. The cursor then moves in a direction and at a speed corresponding to the direction and pressure of the operator's touch.
Although pointing devices that comprise an elastomeric pressure sensitive pad are ergonomically desirable, joysticks have already achieved widespread consumer recognition and acceptance. A low cost, accurate force-sensing joystick for use in consumer electronics is, therefore, desirable. Force-sensing joysticks typically use strain gauge sensors mounted on a portion of the device that bends under an applied force. For example, International Patent Application PCT/US90/06831 of Rutledge and Selker for "Analog Input Device Located in the Primary Typing Area of a Keyboard" describes a strain gauge sensor positioned on a cantilever arm that bends as force is applied to a combined alphanumeric key/joystick. Such strain gauge sensors are relatively expensive and, therefore, increase the cost of a computer utilizing a pointing device incorporating such sensors.
Another disadvantage of current force-sensing joysticks is temperature sensitivity. As the ambient temperature changes, mechanical parts of the joystick assembly expand or contract. This dimensional change can induce in the joystick assembly stresses that are detected by the force sensor. For example, U.S. Pat. No. 5,231,386 to Brandenburg et al. for "Keyswitch-Integrated Pointing Assembly" describes a combined alphanumeric key/joystick in which the key/joystick rests on four pads, each pad activating a sensor. The key/joystick is held in contact with the sensors by rigid fasteners. The stress in the sensors changes in response to a change in ambient temperature. Compensation schemes that correct for temperature sensitivity can add complexity and cost to the joystick. The problem of temperature instability is more acute in portable devices used in a wide variety of locations and environments. Likewise, the strain gauge device described in application PCT/US90/06831 shows tremendous sensitivity to temperature variations.